Articles manufactured from poly(alkylene terephthalates) have many valuable characteristics, including strength, toughness, solvent resistance, high gloss, and the like. These articles may be fabricated by a number of well known techniques, including injection molding, roto molding, blow molding, extrusion, and the like, depending on the shape of the desired product.
Certain of these techniques, in particular, blow molding and extrusion, require that the molten poly(alkylene terephthalate) have a suitably high melt viscosity, e.g., in excess of 10,000 poises, to prevent collapse or blow-outs in the soft preformed state. It has been found that poly(alkylene terephthalates) of such high melt viscosity are obtained only with great difficulty in the conventional bulk melt polymerization processes generally used to prepare the polyester.
It is easier to achieve high melt viscosities if a small amount of a tri- or higher functional ester-forming branching component is included in the polyester, and still easier if the branched copolyester is subjected to solid state polymerization, i.e., heating particles of the resin at a temperature of above 150.degree. C. and below the sticking point of the particles, in an inert atmosphere or under a vacuum.
It has now been discovered that branched copoly(alkylene terephthalates) of high melt viscosity can be obtained in reduced reaction period if a small amount of an aromatic (poly-) or (copoly-)carbonate resin, e.g., a bisphenol-A (poly-)carbonate, or a bisphenol-A/tetrabromobisphenol-A (copoly-)carbonate, is intimately blended with the branched copolyester before solid state polymerization.
The amounts of added aromatic (poly-) or (copoly-)carbonate can have a similar effect in different degrees. Low concentrations, e.g., 0.1 to 5 percent by weight, have a minimum effect on product properties, while larger concentrations, e.g., up to 15 or even 20 percent by weight, in addition to reducing the reaction time, lead to interesting and important products of lower crystallinity, higher impact strength and increased flexibility.